The present invention relates to a zirconia sintering body having a semiconductive property while high strength of the zirconia sintering body is maintained, and concretely suitable for uses in which it is necessary to remove static electricity from jigs and tools used in manufacturing processes of a semiconductor device, a magnetic head, electronic parts, etc. and separating claws, etc. used in a tape guide and an image forming apparatus.
A ceramic sintering body constructed by alumina, zirconia, silicon nitride, silicon carbide, etc. as principal components used as materials of structural parts conventionally has high strength and high hardness and also has an excellent heat resisting property and an excellent anticorrosive property. Accordingly, the ceramic sintering body is used in various fields, but a zirconia sintering body is used in uses requiring particularly excellent mechanical strength and sliding characteristics.
The zirconia sintering body is formed by a high insulating material. Therefore, it is necessary to set a volume specific resistance value (hereinafter, briefly called a resistance value) of the zirconia sintering body to be equal to or smaller than 10.sup.9 .OMEGA..multidot.cm so as to use this zirconia sintering body in uses in which an electrostatic removing action is required in a conveying arm and a pincette for gripping a wafer used in a semiconductor manufacturing device, etc., a separating claw used in an image forming apparatus such as a printer, etc., a tape guide used to convey and guide a tape-shaped body such as a magnetic tape, etc. Therefore, a conductivity giving agent is included on trial in the zirconia sintering body to reduce the resistance value.
For example, Japanese Laid-Open (KOKAI) Patent No. 60-103078 discloses a conductive zirconia sintering body which is mainly constructed by ZrO.sub.2 stabilized by Y.sub.2 O.sub.3 and MgO and includes at least one kind of carbide among carbides such as TiC, TaC, WC, etc. as a conductivity giving agent and has a resistance value from 0.5 to 60.times.10.sup.-3 .OMEGA..multidot.cm.
However, the resistance value of the above conductive zirconia sintering body is excessively low so that static electricity is removed at once when the static electricity is escaped. Accordingly, a problem exists in that discharge is caused by atmospheric friction with an extra-high voltage. Therefore, for example, there is a fear that recording contents of the magnetic tape are broken when the tape guide is formed by the above zirconia sintering body and the static electricity caused by a sliding movement of the tape guide with respect to the magnetic tape is removed.
When such a zirconia sintering body is manufactured, a special device is also required since a burning operation must be performed in a nonoxidizing atmosphere. Further, a problem exists in that manufacturing cost is increased since a raw material itself of the above conductivity giving agent is expensive.
An object of the present invention is to provide a semiconductive zirconia sintering body able to be burned in an oxidizing atmosphere and able to be cheaply manufactured and escape static electricity at a suitable speed without greatly reducing mechanical characteristics of zirconia.
Therefore, in consideration of the above object, a semiconductive zirconia sintering body of the present invention is characterized in that the semiconductive zirconia sintering body is constructed by 60 to 90 weight % of ZrO.sub.2 including a stabilizing agent, and 10 to 40 weight % of one kind or more of oxides of Fe, Co, Ni and Cr as conductivity giving agents and has a volume specific resistance value from 10.sup.5 to 10.sup.9 .OMEGA..multidot.cm.
Namely, the semiconductive zirconia sintering body of the present invention includes an oxide of one kind or more among oxides of Fe, Co, Ni and Cr as conductivity giving agents. Accordingly, it is found that these conductivity giving agents constitute a grain boundary phase, and the sintering body can have a semiconductive property ranging from 10.sup.5 to 10.sup.9 .OMEGA..multidot.cm in volume specific resistance value without greatly reducing the strength of zirconia.
Therefore, static electricity can be escaped at a suitable speed. Accordingly, when an object coming in contact with the zirconia sintering body tends to be electrically influenced, the static electricity can be removed from the object without breaking this object.
Further, each of the above conductivity giving agents is constructed by an oxide so that the conductivity giving agents can be burnt in an oxidizing atmosphere. Accordingly, no special device is required to burn the conductivity giving agents. Further, these conductivity giving agents can be cheaply obtained so that the zirconia sintering body can be simply and cheaply manufactured.
However, when a content of the above conductivity giving agents is smaller than 10 weight %, effects for reducing the resistance value are small. Conversely, when this content is greater than 40 weight lo the resistance value is smaller than 10.sup.5 .OMEGA..multidot.cm so that the static electricity tends to be escaped at once. Therefore, there is a fear that discharge is caused by atmospheric friction with an extra-high voltage. Further, mechanical characteristics (flexural strength, fracture toughness value, hardness, etc.) of the sintering body are greatly reduced so that no original mechanical characteristics of zirconia can be shown.
Therefore, it is important to set the content of the conductivity giving agents to 10 to 40 weight % and preferably 20 to 30 weight %.
ZrO.sub.2 as a principal component is partially stabilized by a stabilizing agent such as Y.sub.2 O.sub.3, MgO, CaO, CeO.sub.2, etc.
Concretely, when Y.sub.2 O.sub.3 is used as a stabilizing agent, Y.sub.2 O.sub.3 is added to ZrO.sub.2 in a range from 3 to 9 mol %. When MgO is used as a stabilizing agent, MgO is added to ZrO.sub.2 in a range from 16 to 26 mol %. When CaO is used as a stabilizing agent, CaO is added to ZrO.sub.2 in a range from 8 to 12 mol %. When CeO.sub.2 is used as a stabilizing agent, CeO.sub.2 is added to ZrO.sub.2 in a range from 10 to 16 mol %. If the stabilizing agents are added in these ranges, a zirconia (tetragonal zirconia and cubic zirconia) amount except for monoclinic zirconia with respect to all zirconia amounts can be set to be equal to or greater than 90%. Therefore, it is possible to restrain a strength addition caused by including the conductivity giving agents and realize a flexural strength equal to or greater than 580 MPa, a high fracture toughness value equal to or greater than 5 MPam.sup.1/2 and a high Vickers hardness equal to or greater than 9.5 GPa.
Namely, a crystal state of zirconia is constructed by three states composed of cubic, tetragonal and monoclinic states. The tetragonal zirconia is particularly transformed to the monoclinic zirconia in phase by stress induced transformation with respect to external stress. Fine microcracks are formed around the zirconia by volume expansion caused at this time so that progress of the external stress can be prevented. Accordingly, strength of the zirconia sintering body can be increased.
Therefore, if a thin conveying arm and a pincette for gripping a wafer used in a semiconductor manufacturing device, etc., a separating claw used to separate paper from a roller in an image forming apparatus such as a printer, etc., a tape guide, etc. used to convey and guide a tape-shaped body such as a magnetic tape, etc. are formed by this zirconia sintering body, no zirconia sintering body is worn and damaged for a short period so that the zirconia sintering body can be suitably used for a long period.
An average crystal particle diameter of ZrO.sub.2 in the zirconia sintering body ranges from 0.3 to 1.0 .mu.m and preferably ranges from 0.4 to 0.6 .mu.m.
An X-ray diffraction intensity of the monoclinic zirconia and X-ray diffraction intensities of the tetragonal zirconia and the cubic zirconia are respectively measured by X-ray diffraction to calculate the zirconia amount except for the monoclinic zirconia with respect to all the zirconia amounts in the zirconia sintering body. This zirconia amount is calculated by the following calculating formula. ##EQU1##
X.sub.m :zirconia amount (%) except for monoclinic zirconia with respect to all zirconia amounts PA1 I.sub.m : X-ray diffraction intensity of monoclinic zirconia PA1 I.sub.t : X-ray diffraction intensities of tetragonal zirconia+cubic zirconia
Further, there is a fear that Al.sub.2 O.sub.3, MnO, SiO.sub.2, Na, Fe, etc. are mixed as impurities with raw material powder in a manufacturing process. However, these impurities may be included in the raw material powder if these impurities have a weight % equal to or smaller than 2.0.
When such a semiconductive zirconia sintering body is manufactured, ZrO.sub.2 powder having an average particle diameter from 0.5 to 1.0 .mu.m, Y.sub.2 O.sub.3, MgO, CaO and CeO.sub.2 powders as stabilizing agents, and oxide powder of one kind or more among oxides of Fe.sub.2 O.sub.3, Co.sub.3 O.sub.4, NiO, Cr.sub.2 O.sub.3 as conductivity giving agents are used. Otherwise, hydroxide powder, carbonate powder, etc. changeable to these materials during burning are used. ZrO.sub.2 including the stabilizing agents and the conductivity giving agents are adjusted such that this ZrO.sub.2 has 60 to 90 weight % and the conductivity giving agents have 10 to 40 weight %. These materials are mixed with each other by a dry or wet type. In the case of the wet type, granules can be also made by drying these materials by a spray drier, etc.
A die is filled with the raw material powder formed by the dry type and the granules formed by the wet type. The raw material powder and the granules are then molded in a predetermined shape by a well-known molding means such as a mechanical press molding method, a rubber press molding method, etc. Otherwise, a slurry formed by the wet type is molded in a predetermined shape by a well-known molding means such as an extrusion molding method, an injection molding method, a tape molding method, etc. Thereafter, the molded slurry is burnt for about one to three hours in an oxidizing atmosphere. At this time, when a burning temperature is lower than 1360.degree. C., no molded slurry can be perfectly sintered. In contrast to this, when the burning temperature is higher than 1450.degree. C., sinter over is caused. Accordingly, strength and hardness of the zirconia sintering body can be increased in each of these cases. Therefore, it is important to burn the molded slurry at a temperature from 1360 to 1450.degree. C.
When the zirconia sintering body is manufactured in such a condition, it is possible to obtain a semiconductive zirconia sintering body in which the zirconia amount except for the monoclinic zirconia with respect to all the zirconia amounts is 90% or more, and the flexural strength is 580 MPa or more, and the fracture toughness value is 5 MPam.sup.1/2 or more and the Vickers (Hv) hardness is 9.5 GPa or more, and the volume specific resistance value ranges from 10.sup.5 to 10.sup.9 .OMEGA..multidot.cm.
A coprecipitation method may be used when ZrO.sub.2 and the stabilizing agents are mixed with each other. If this coprecipitation method is used, it is possible to obtain ZrO.sub.2 in which the stabilizing agents are dispersed finely and uniformly.